1. Technical Field
The present invention relates to a method of determining a remaining operating time of a potentiometric measuring probe containing an electrolyte as well as a primary reference element and a secondary reference element, wherein the reference elements are arranged so that an electrolyte deficiency advancing from an opening in the measuring probe arrives at the secondary reference element before it reaches the primary reference element. In addition, the scope of the invention also includes an apparatus for performing the method, as well as the use of the apparatus.
2. State of the Art
A widely used kind of measuring probe for potentiometric measurements of ion concentrations or redox potentials is equipped with a diaphragm of porous material. The diaphragm serves to bring a reference electrolyte and/or a bridge electrolyte, normally in the form of a liquid contained inside the measuring probe, into contact with a test solution. Particularly in chemical or micro-biological process-monitoring and process-control applications, the diaphragm may,be subject to contamination which can falsify the results of the measurements.
Another measuring probe, disclosed in DE 34 05 431 C2, has no diaphragm and is significantly less prone to contamination. It has a housing of an electrically insulating material with at least one enclosed space containing a reference element and an electrolyte. The housing has at least one opening through which the electrolyte can be brought into contact with a liquid solution on the outside of the housing, i.e., with the medium on which a measurement is to be performed. The enclosed space inside the housing is filled with an ion-permeable, micro-porous, high-viscosity polymer substance which, in combination with the electrolyte, forms a filler mass of the measuring probe. This type of construction assures that the electrical potential measured at the reference element is highly constant even if the solutions being measured are strongly contaminated. In addition, the measuring probe can sustain pressure levels significantly in excess of 10 bar.
Measuring probes of the foregoing description are known to have the problem that, as the cumulative operating time of the probe advances, the electrolyte that is initially contained in the polymer substance will to an increasing degree migrate into the test solution, resulting in a progressively spreading electrolyte deficiency in the polymer substance inside the housing. The increasing electrolyte deficiency in the polymer substance is also referred to as the aging process of the measuring probe and produces the undesirable effect that, when the electrolyte deficiency eventually reaches the reference element, there will be a change in the electrical potential measured at the reference element. To avoid the risk of erroneous measuring results, it is therefore necessary to monitor the aging process of the measuring probe. In particular, it should be possible to detect sufficiently in advance when the electrolyte deficiency is approaching the reference element, i.e., at a point when there is still an adequate amount of time left during which the probe can continue to operate.
According to DE 34 05 431 C2, the problem of detecting the advancement of the electrolyte deficiency can be solved by using an electrolyte consisting of a suspension of homogeneously distributed particles of a neutral salt with ions of equal transport number in an aqueous solution of the neutral salt. The polymer substance and the neutral salt suspension together form a gel that has a turbid appearance due to the salt particles in suspension. The state of aging of the measuring probe can be visually detected, as the turbidity disappears progressively with the advancement of the aging process. The reason for the decrease in turbidity is that the suspended neutral salt particles continuously pass into solution until a final state has been reached where there are essentially no suspended particles left, so that as a result the turbidity is strongly diminished. It has been found that in the aging process, a clearly visible boundary develops between a turbid portion of the gel where the neutral salt particles are homogeneously suspended and a comparatively clear portion where the neutral salt particles have passed into solution. The state of advancement of the boundary from the opening in the housing towards the reference element can be determined through visual observation. Based on a current position and speed of advancement of the boundary, it is possible to draw conclusions about the current state and speed of aging and thus predict the remaining operating time of the measuring probe.
However, the measuring probe according to DE 34 05 431 C2 has several drawbacks. To monitor the state of aging and determine the remaining operating time of the probe, it is necessary to be able to clearly see inside the enclosed space of the measuring probe. This precludes the use of a non-transparent material for the housing, and it also presents a problem with a transparent housing if the latter becomes contaminated by surface deposits. A further severe problem occurs if the gel in the enclosed space becomes discolored or contaminated, e.g., by the infusion of colored substances or infiltration of dirt particles from the test solution, which could make it practically impossible to visually detect the boundary of the electrolyte deficiency. It also has to be counted as a drawback that, in order to make the boundary visible, the electrolyte needs to be a suspension of homogeneously distributed particles of a neutral salt with ions of equal transport number in an aqueous solution of the neutral salt, a condition that excludes other kinds of electrolytes from being used in the measuring probe.